Apr 09, 2012
Season 1 Episode 4
The Cave of Swallows in Mexico is a 400m vertical shaft, deep enough to engulf the Empire State Building. The Lechuguilla cave system in the USA is 193km long with astonishing crystal formations. Caves are remarkable habitats with equally bizarre wildlife. Planet Earth gets unique access to a hidden world of stalactites, stalagmites, snotites and troglodytes.